Substitute dominants replace existing
dominant seventh chords, creating a resolution of down a half-step instead of forward through the
cycle of fifths.
This process is sometimes refered to as tritone substitution, because the roots of the original dominant and the substitute dominant will be a tritone apart,
and because both chords use the same tritone for guide tones.

Substitute dominants share the same
guide tones as the replaced dominant chord.
For G7, the primary dominant of C,
the guide tones are B (the third of G7, ti of the key)
and F (the seventh of G7, fa of the key).
These notes are the guide tones of Db7,
with F now being the third,
and B now being Cb,
the seventh of Db7.
Notice the roots G and Db are a tritone apart.